The generation of sources of radiation in the ultraviolet, visible, infrared and up to the terahertz, at a distance, by nonlinear optical interaction or photocurrent generation of two, or multiple, short pulses is extremely sensible to spatial and temporal overlap of the pulses in the interaction medium. The occurrence and efficiency of the generation process is highly dependent on the ability to respect superposition conditions of the two beams, which may be complex, especially if the interaction takes place at long distance or/and at high intensities.
A current approach in nonlinear optical interaction, through four-wave mixing between two pulses of slightly different wavelengths for example, is based on splitting a laser beam into two different arms, in which one pulse is frequency doubled and the second harmonic recombines with the other fundamental pulse in a filament where the difference in frequency results in IR radiation (see FIG. 1). A first one of the arms includes a delay line, which can be made by mirrors (M) on a translation stage for instance, in order to achieve temporal overlap of the two beams.
A limitation in current systems and methods is the instability of the UV, VIS, IR and/or THz source generation due to the difficulty of using an interferometer for spatial-temporal superposition of two-color pulses. As well known to people in the art, in such a configuration, the precise alignment of both arms is critical and even if they seem exactly collinear in the range of the setup, the spatial overlap decreases as the distance increases. Also, vibrations and atmospheric density fluctuations greatly reduce the distance where the spatial-temporal overlap can be produced, typically to a few meters, and causes large fluctuation of the generated optical source, be it a UV, VIS, IR, or a THz source. Another limitation is due to the use of dichroic mirrors that divide and recombine the beams because they work only for a given laser configuration and need to be replaced for different input parameters such as the pulse wavelength, requiring further alignments.
There is still a need in the art for a system and a method to spatially and temporally overlap multiple-color pulses for long distance tunable UV, VIS, IR and/or THz source.